Processing substrates with plasma is widely used in a broad range of industries. A disk processing system using a plasma-enhanced chemical vapor deposition (PECVD) source, i.e. a PECVD system, is one example of a plasma-based enabling technology that may be used to coat the surface of media storage disks with a protective layer of carbon.
FIG. 1A illustrates a partial plan view of one type of processing system. The process system illustrated in FIG. 1A is a PECVD processing system known as the NCT Mark V Process Station (the “NCT unit”), which is manufactured by Intevac, Inc. of Santa Clara, Calif. FIG. 1B illustrates a conceptual operation of the NCT unit. During operation of the NCT unit, neutral gas molecules 100 enter the chamber 110 from a gas source 114 located behind gas diffuser 118. A filament 119 emits electrons 102 toward anode 116 that excite the neutral gas molecules 100 to generate plasma. Ions 104 are deposited onto disk 106 to coat the substrate surface with a protective diamond like carbon (DLC) coating.
The NCT unit design inhibits the backflow of plasma from chamber 110 into gas source 114 using a permanent magnet 112 positioned within an enclosure 120. The permanent magnet 112 is formed from NdFeB 38 magnetic material having a diameter of 1.0 inch and a length of 2.0 inches. The permanent magnet 112 generates a magnetic field that defines flux path 122. Flux path 122 moves through gas diffuser 118 in a direction that repels electrons 102 and ions 104 from entering where they might cause undesirable pre-ignition of neutral gas molecules 100.
While the NCT unit inhibits the backflow of electrons 102 and ions 104, the magnetic field may also distort ion distribution within chamber 110. The magnetic field may enter the chamber 110 directly by passing around the gas diffuser 118, which is spaced a distance of about 3.22 inches from the permanent magnet 112 and has a thickness of about 0.1 inch. Alternatively, the magnetic field may enter the chamber 110 indirectly by straying as it travels between the gas diffuser 118, which has a diameter of 2.25 inches, and the enclosure 120, which has a diameter of 8.00 inches. Magnetic field that enters the chamber 110 may distort ion distribution therein.
FIG. 1C illustrates the negative impact that ion distortion within chamber 110 may have on the carbon coating process. Ion distortion generates coating thickness variation through uneven ion distribution and unwanted ion deposition onto anode 116 and chamber magnets 124, which may lead to escalating variation in coating thickness variation. As a result, standard deviation in carbon coating thickness across the entire disk face may gradually increase from a cycle-start standard deviation 130 of, for example, about 5% of nominal substrate coating thickness at the beginning of a production cycle to a cycle-stop standard deviation 132 of, for example, about 10% of nominal substrate coating thickness at the end of a production cycle. Increasing coating variability ultimately translates into degraded performance of the media storage disk.